The present invention relates to a CT image reconstruction method, a CT apparatus, and a program. More particularly, the present invention relates to a CT image reconstruction method, a CT apparatus, and a program for interpolating back projection data, which represents each slice plane, using projection data items produced by scanning a subject, and reconstructing a CT image on the basis of the back projection data.
In X-ray CT apparatus, a subject is scanned with an X-ray fan-shaped beam, and an X-ray detector having numerous (for example, 1000) detector elements set in array detects X-rays transmitted by the subject. The subject is intermittently or continuously moved in the directions of the body axis according to an axial or helical scan technique, whereby all projection data items representing a specific scan field are acquired and stored. Back projection data required for reconstruct an image is read or interpolated for each slice plane, and a CT image of the subject is reconstructed based on the back projection data.
In recent years, along with the tendency towards an X-ray detector having more channels and more detector arrays (16 detector arrays), the frequency of occurrence of a defect such as poor sensitivity in a detector element or detector data (which refers to projection data) has increased. If defective data is used to reconstruct an image, a tomographic image is affected by a ring artifact or the like. The defective data should therefore be compensated by other detector data.
In order to compensate defective data caused by microscopic discharge of an X-ray tube, a conventionally known X-ray CT apparatus interpolates projection data, which is contained in a view that cannot be produced normally, using projection data items contained in views preceding or succeeding the view in an X-axis direction, projection data items contained in an opposite view, or projection data items produced at the same view angle as the view angle, at which the view is produced, during scans preceding or succeeding in a body-axis direction the scan during which the view is produced (Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-116841 (Claims 1, 3, and 4)
According to the technique of interpolating defective data itself, when back projection data representing each slice plane whose image is reconstructed is interpolated, pieces of positional information (channel numbers and view angles) on normal projection data items used to interpolate the defective data are lost. Therefore, the back projection data contains inconsistent positional information.
The above drawback will be described with reference to drawings. Referring to FIG. 4, reference numeral 40 denotes an X-ray tube, reference numeral 90 denotes an X-ray detector, and reference numeral 100 denotes a subject. Assume that projection data g (CH3,Vj) produced by a channel CH3 and contained in a view Vj is defective. Conventionally, projection data items g(CH2,Vi) and g(CH4,Vk) contained in views Vi and Vk preceding or succeeding the view Vj in which the defective data is contained are summated at a predetermined ratio in order to produce interpolated data g′(CH3,Vj).
However, the defective data g(CH3,Vj) stems from a signal having transmitted along a line Lj. The projection data items g(CH2,Vi) and g(CH4,Vk) stem from signals having transmitted along lines Li and Lk respectively. Strictly speaking, the signals are different from one another in terms of a path along which a signal is transmitted by the subject 100 (a region).
Referring to FIG. 3, back projection data h(CH3,Vj) is interpolated in order to represent each position on each slice plane Si for the purpose of image reconstruction. Conventionally, a count value alone is taken into account. Interpolated data g′(CH3,Vj) resulting from interpolation is used as projection data, which is supposed to be produced by a channel CH3 and contained in the view Vj, in order to interpolate back projection data. The attributes (especially, pieces of positional information) of the projection data items g(CH2,Vi) and G(CH4,Vk) that are used to interpolate the data g′(CH3,Vj) are not reflected on production of back projection data. This deteriorates the quality (faithfulness) of a reconstructed image.